This invention relates generally to printing an image on a substrate. More particularly, it relates to an iterative technique of scanning and printing an image on the substrate in successive passes.
There is a substantial body of technology devoted to producing the most accurate images from a printer. Typically, complicated algorithms such as error diffusion and dithering are used so that a printer can come as close to the desired image as possible. Yet despite the body of prior art, to date most printers are basically blind to the image which is actually produced. The printer is limited to a "best guess" as to the appropriate amounts of ink to deposit on the substrate. While the "best guess" has been developed according to the observations of the engineers who developed the printer, it usually represents a compromise between a relatively limited number of images used during printer development. In recognition that some of these compromises may be unacceptable, some printing technologies allow a trained operator to tune parameters based on the actual printer output.
The problems experienced in the prior art are visually depicted in FIG. 1. Most commercially available printers can vary the intensity of a pixel over a limited range. Error diffusion gives the appearance of a full gray scale by varying the ratio of dark pixels, like a Monet painting. Based on the desired image, a calculation is made to determine whether a particular inkjet from an inkjet printhead or wire from a impact printer should fire when the printerhead moves over that particular location on the paper. The desired density of the real image is compared to estimated density of the ink on the paper if the ink was fired at that location. When the inkjet nozzle is fired, the realized density varies widely from the precalculated expected density as shown by the horizontal streaks in FIG. 1. This variance in realized density makes the printed image a noisy image.
The prior art has provided limited optical feedback to a printer. For example, it is known to measure the light produced by an LED or a laser printer to provide corrections during the actual printing operations. In response to the amount of light sensed, adjustments are made by changing the drive current to the LEDs or laser or changing the duration of an exposure period. However, this feedback does not measure what is actually produced on the paper, but provides a means for more accurately controlling the amount of light emitted. Similar feedback techniques exist in the copier art. Another example of combined optical feedback and printing is found in the prior art for registration purposes. Where successive plates of color are used to create an image, for example, in a screen printer, it is known to locate the image printed for registration purposes for the printing of subsequent images. However, no provision is made for correction between the deviation between the desired image and that which is actually printed during a successive printing stage.
The present invention provides a means of correcting printer output based on optical feedback.